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I have readopted JDO recently. The cause ? Google App Engine’s big table storage model that makes JDO, a storage agnostic framework for persistence, ideal for such implementations.

Some very useful links related to Java on the GAE are following

1. JRE Class White List A Java App Engine application’s access to the classes in the Java standard library (the Java Runtime Environment, or JRE) is limited to the classes displayed in that link.
2. Will it Play in App Engine. This google group post aggregates all known java technologies and frameworks with commenting on whether or not they are applicable to the appengine JVM runtime. As we have discussed before limitations to certain JEE classes will stop many of the well adopted frameworks to operate.
3. Web Services Connector. Due to limitations in the java.net core library many web services client frameworks fail to operate in appengine. Frameworks such as axis, axis2, JAX-WS do not have compatibility since they use not authorized classes. A simple and lightweight alternative is   WSC from people in salesforge.com that generates a simple jar client provided the wsdl document services definition. Another more nasty alternative is to implement the calls to the service by yourself and use JAXB or JIBX or even xmlnbeans to marshal/unmarshal the request/response documents.